Power electronic modules are widely used components in which multiple of power electronic switches or devices are placed in a single module. The switches of a power electronic module are wired inside the module in specified manner such that power electronic modules can be used in different circuit structures. Such circuit structures are, for example, power stages of different power converters. For that purpose, the power electronic modules may comprise different half-bridge, full bridge or other bridge topologies in which controllable switch components are internally connected with power diodes. The power electronic modules comprise also terminals, such as control terminals and power terminals that allow connecting the modules to other required circuitry and possibly to other modules.
The components inside a power electronic module are typically mounted on a substrate that is thermally connected to the base plate of the module. The base plate is a metallic piece integrated to the bottom of the module and it is intended to be attached to a surface of a cooling member, such as heat sink. The semiconductor switches inside the modules generate heat when the switches are operated. The switched currents can be over hundreds or even thousands of amperes and the voltage blocking ability of the power semiconductors of the module is several thousand volts. These semiconductor switches are further operated at a relatively high frequency of several thousand Hertz.
A proper thermal design is crucial for a reliable operation of power electronic modules. A violation of the temperature ratings can lead to a reduced safe operating area and consequently a sudden device failure or to a reduced operational lifetime. For example, IEC 60747-9 gives a range of temperature ratings for IGBTs like storage temperature, case temperature and virtual junction temperature. To keep the temperature of the module at a tolerable range, it is known to attach the module to a heat sink. This is performed by attaching the planar surface of the base plate or of the substrate to a corresponding planar surface of a heat sink. The heat transfer between the bottom surface of the module and the heat sink is enhanced by using a thermal interface material (TIM). Such material or layer is placed between the surfaces of base plate and heat sink or surfaces of substrate and heat sink if the module is without a base plate.
Thermal connection between two surfaces depends on several properties including their surface roughness (Ra) and surfaces' planarity. In practice the contact of two surfaces is imperfect and there are gaps filled with air in between them. Because air is poor thermal conductor the contact thermal resistance (Rth) can be reduced by making the contact surfaces perfectly smooth and planar (very expensive) and/or by replacing the air by a better thermal conducting substance.
Specific materials have been developed to fill air gaps between the surfaces in contact and to reduce contact thermal resistance. These thermal interface materials are available in different physical forms like dispensable grease or paste, various thickness rubber like pads, metal foils and so on. These materials are typically mixtures that include carrier or substrate chemical (like silicone oil or rubber) that has been filled with higher thermal conducting material like boron nitride or carbon nanotubes. Some TIMs have also phase changing property, which may be beneficial for example in cyclic operation applications. This type of TIM is proven to work well in many applications where the contact surface area is relatively small and planar like with cooling of CPUs. Another working solution for CPUs and similar small footprint and planar surfaces is the use of graphite thermal interface materials. Known graphite TIMs have high temperature conductivity and are suitable for small footprint devices due to hardness of the material.
However, the base plate footprint size of PE modules is several times larger compared to CPU's footprint size. Also the base plate surface profile is far from planar in power electronic modules. The base plate's deviation from flatness may exceed 0.1 mm (>100 μm) over 50 mm length. This initial out-of-planarity causes uneven contact pressure at module mounting. Temperature gradient, power cycling and coefficient of thermal expansion differences within the materials of PE module cause dynamic variation to the thermal interface shape. This dynamic behavior causes complex deformation scenario to the thermal interface, which loads the TIM, and in many cases destroys TIM's capability to carry out the heat conduction function. For greases this means “pump-out” where grease is leaking away and for foils this means rupture and dropping pieces of foil. The above power cycling refers to uses of the power electronic module in which the loading of the module is periodical. The periodical or cyclic loading means further that the temperature of the module changes according to the cyclic loading.
As during the power cycling shear stresses are applied to the thermal interface material, the thermal interface material breaks down and loses its ability to conduct the heat effectively from the base plate to the cooling arrangement. This in turn leads to increase of temperature of the switching components and to premature breakdown of the components.